At the surface of the alveolar epithelium in the lung fies a liquid, the alveolar lining layer, consisting of an aqueous subphase covered by a film of pulmonary surfactant. It is difficult to verify it morphologically because the fining layer is primarily water and quite thin. Cryo-SEM/stereo techniques are used to visualize the alveolar lining layer and measure the thickness of the fluid layer that lines the lung and coats the lung epithelium at the air-lung interface.